1. Field of the Invention
The present invention relates to a novel liquid crystal composition. In optical response to an applied voltage, the novel liquid crystal composition of the present invention exhibits an optical response (V-letter-shaped optical response) which is symmetrical in a positive voltage region and a negative voltage region and which involves a small hysteresis or is free of hysteresis. Further, the liquid crystal composition of the present invention is excellent in alignment and alignment stability. The liquid crystal composition of the present invention can materialize an active matrix liquid crystal display device having high display qualities and high display reliability.
2. Prior Art of the Invention
A liquid crystal display device (LCD) has been widely used as a flat panel display and is taking the place of a conventional Braun tube (CRT) display, mainly in portable machines and equipment. Along with the recent expansion of the functions of personal computers and word processors and with the recent increase in the capacity of data processing, LCD is also required to have higher functions, that is, to have functions such as a large display capacity, a full-color display, a wide viewing angle, a high-speed response and a high contrast.
As a liquid crystal display method (liquid crystal driving method) to comply with such requirements, an active matrix (AM) display device has been proposed and, to some extent, is practically used. In the AM display device, thin film transistors (TFT) or diodes (MIM) are formed such that each element is constituted of one transistor or diode and one pixel on a display screen and a liquid crystal is driven for one pixel independently of another.
The above display method has had problems that decreasing a cost is difficult due to a low production yield and that forming a large-sized display screen is difficult. However, the problems are being overcome little by little, and the above display method is about to surpass a conventional mainstream STN display method and is about to overtake CRT due to its high display quality.
However, the above AM display device has the following problems due to the use of a TN (twisted nematic) liquid crystal as a liquid crystal material.
(1) A TN liquid crystal is a nematic liquid crystal, and the response speed is generally low (several tens ms), so that no good image quality can be obtained in the display of video frames.
(2) A twisted state (twist alignment) of liquid crystal molecules is used for displaying, and the viewing angle is therefore narrow. In displaying with a gray scale in particular, the viewing angle becomes sharply narrowed. That is, the contrast ratio, the color or the like changes depending upon viewing angles to a display screen.
For overcoming the above problems, there have been, in recent years, proposed AM panels which use a ferroelectric liquid crystal or an anti-ferroelectric liquid crystal in place of the TN liquid crystal (Japanese Laid-open Patent Publications Nos. 5-249502, 5-150257 and 6-95080). At present, however, the following problems remain to solve for the practical use of these liquid crystals.
(A) A ferroelectric liquid crystal has spontaneous polarization. An image sticking is liable to occur due to constant presence of the spontaneous polarization.
In displaying in a surface stabilization mode with a ferroelectric liquid crystal, it is very difficult to perform a gray-scale display since only a binary display of black and white is possible in principle. For the gray-scale display, a special artifice is required (for example, use of a ferroelectric liquid crystal device using monostability; Keiichi NITO et al., SID ""94, Preprint, p. 48), and it is required to develop a high technique for practical use.
(B) An anti-ferroelectric liquid crystal is free from the image sticking problem described in the above (A) since it has no spontaneous polarization.
In the AM driving, there is at least needed a liquid crystal material which can be driven at 10 V or lower. However, the anti-ferroelectric liquid crystal generally shows a high threshold voltage, and its driving at a low voltage is therefore difficult. Further, it has another problem that the gray-scale display is difficult to perform since its optical response involves a hysteresis.
In recent years, the present inventors have found a liquid crystal material which can highly possibly overcome the above problems (Japanese Laid-open Patent Publication No. 8-337555). The above material can be driven at a voltage of 10 V or lower and is free of a hysteresis in optical response. In optical response to a charged voltage, the above material exhibits an optical response which is symmetrical in a positive voltage region and a negative voltage region and which involves a small hysteresis or is free of hysteresis, so that the above material is optimum for active matrix driving.
However, active-matrix-driving experimental liquid crystal display devices prepared from the above material have a problem that even a device which has an excellent initial alignment state and gives a high image quality comes to show a deterioration in image quality with the passage of time.
The present inventors have made further studies for overcoming the above problem.
Generally, in measurement of a liquid crystal for physical properties, a test cell in which the liquid crystal is injected is used, the liquid crystal is measured for a series of properties by applying an AC voltage, such as an optical response by applying a triangular wave voltage, and the liquid crystal is finally measured for a tilt angle by applying an DC voltage. The test cell used for the above measurements of physical properties are stored under predetermined conditions for a predetermined period of time, and then disposed of.
In the above measurements using the test cell, the measurements take approximately several hours, and in such a case, there is caused no special problem.
When data obtained by the above measurements were analyzed, data of some test cells involve some questions. These test cells were therefore again used for the measurements. However, the liquid crystals of the test cells changed in alignment state to a great extent, and no general physical property data could be obtained. That is, they were not measurable for physical properties.
The present inventors have therefore made further studies for causes thereof.
First, the present inventors have made studies for a method to restore the alignment state of liquid crystals of the stored test cells. However, the restoration has not at all been possible when general means such as application of a voltage are employed. Therefore, the liquid crystals have been again melted and treated for re-alignment, while it has been impossible to completely reproduce the initial measurement data.
Further, behaviors of a change in alignment state have been studied.
First, it has been found that when the test cells are allowed to stand after the measurement of physical properties made by applying AC voltage, the alignment state changes. When the test cells are allowed to stand without connecting electrodes on both surfaces of each test cell, there are caused some differences in the rate of a change in alignment state or the behavior of the change in alignment state among the test cells for a time period practical for the test method. That is, it has been found that the reproductivity thereof is insufficient. Further, when the test cells are stored for a long period of time, there is found no differences in change in alignment among the test cells. In contrast, when the test cells were allowed to stand in a state where electrodes on both surfaces of each test cell are connected to each other, there is caused a moderate change in alignment state, and the reproductivity comes to be sufficient among the test cells. It has been found that an alignment change which the test cells shows after standing for a long period of time is similar to the result obtained when the test cells are allowed to stand for a long period of time without connecting the electrodes on both the surfaces.
On the basis of the above results, it has been assumed that these liquid crystal materials cause a large change in alignment state when only injected to test cells and allowed to stand, and that the above change shows a spontaneous shift toward a more stabilized state. As described already, further, the liquid crystal display device which has once had the above change cannot be completely restored to its original state even if it is heated to form a uniform phase (Iso) and treated for re-alignment.
Under the circumstances, it has been an essential object to discover a material which does not cause the above phenomenon.
It is an object of the present invention to provide a liquid crystal material which shows an optical response optimum for active matrix driving and which has excellent alignment stability without causing a change in alignment with the passage of time.
According to the present invention, the above object of the present invention is achieved by a liquid crystal composition comprising anti-ferroelectric liquid crystal compound (1) selected from liquid crystal compounds of the following formula (1) and a ferrielectric liquid crystal compound (2) or a racemic compound (2xe2x80x2) thereof, wherein the liquid crystal composition satisfies the following requirements (a) to (c),
(a) the compositional ratio of the anti-ferroelectric liquid crystal compound (1) and the ferrielectric liquid crystal compound (2) or the racemic compound (2xe2x80x2) thereof is in the range of from a value which is on the anti-ferroelectric phase side and is apart from a value of a boundary a compositional ratio by 5 mol % to a value which is on the ferrielectric phase side and is apart from the value of the boundary compositional ratio by 25 mol %, wherein the boundary compositional ratio is obtained on the basis of a liquid crystal phase diagram prepared on the basis of conoscopic image observations obtained by changing the mixing ratio of the anti-ferroelectric liquid crystal compound (1) and the ferrielectric liquid crystal compound (2) or the racemic compound (2xe2x80x2) thereof,
(b) in an optical response to an applied voltage, the liquid crystal composition exhibits an optical response which is symmetrical in a positive voltage region and a negative voltage region and which is free of hysteresis or involves a small hysteresis, and
(c) the liquid crystal composition has excellent alignment stability, 
xe2x80x83wherein R1 is a linear alkyl group having 8 to 11 carbon atoms, X1 is a hydrogen atom or a fluorine atom, A is xe2x80x94CH3 or xe2x80x94CF3, m is an integer of 1 or 3, n is an integer of 1 or 2, and C* is an asymmetric carbon atom.